Detailed studies on the pharmacokinetics of d-methadone, 1-methadone and the racemic drug in animals and humans are in progress. A gas chromatographic assay for the measurement of the parent drug and the major metabolite has been developed. The half-life (t 1/2) of each isomer is the same in the mouse (40-50 minutes). In mice which have been given one or the other isomer (500 microgm i.p.) approximately 20% of the d-isomer is excreted unchanged in the urine in 200 minutes but little or no unchanged 1-methadone appears in the urine in this time. In the perfusate of the isolated perfused rat liver the t 1/2 of the 1-isomer is about 120 min and the t 1/2 of the d-isomer is approximately 70 minutes. In the bile of the isolated perfused rat liver new di-hydroxylated metabolites have been identified by gas chromatography and mass spectrometry. Studies on the effect of the racemic drug and each isomer on the CO2-induced increase in respiratory minute volume in healthy volunteers are also in progress using a double blind technique. The respiratory effects of a single dose of methadone last form 72-120 hrs. Pupillary changes, on the other hand, return to normal within 48 hrs. The blood half-life (t 1/2) of methadone in some subjects appears to follow a biphasic decline with a t 1/2 of eight hrs. for the first phase and a t 1/2 of about 18-40 hrs. for the second phase. A new and highly specific radioimmunoassay for methadone has been developed and efforts are in progress to develop a stereospecific radio immunoassay.